The present invention relates to a magnetic recording medium suited for a perpendicular magnetic recording system.
The perpendicular magnetic recording system performs recording in a direction perpendicular to the plane of a recording medium film, and it is a system which is suitable for enhancing a recording density because a demagnetizing field within each bit in the case of high density recording is low. As magnetic recording media which are used for this purpose, there are Co-based alloy films such as Co-Cr, Co-V, Co-Mo, Co-W, Co-Re, Co-O, Co-Cr-Rh, Co-Cr-Ru and Co-Ni-O films. Any of these Co-based alloys has a hexagonal close-packed (h.c.p.) crystal structure, and possesses the merit that fine crystal grains constituting the thin film are liable to C-axis orientation. In order to enhance magnetic recording characteristics, the degree of C-axis orientation of the thin film needs to be raised.
The perpendicular magnetic recording media presently in use are such that the Co-based alloy film is adhered to a non-magnetic substrate directly or through a soft magnetic thin film of permalloy (an Ni-Fe alloy whose principal ingredient is Ni and which exhibits a high magnetic permeability) or the like. A plastics film of polyimide, polyethylene terephthalate or the like, an Al or glass plate, etc. are employed as the non-magnetic substrates. In case of forming the Co-based alloy film on such a substrate, this Co-based alloy film must have a good adhesion with the substrate and a high degree of C-axis orientation. However, in a case where the Co-based alloy film is directly adhered to the plastics film or the glass plate by an evaporation method, there are the problems that the bond strength is insufficient, so the Co-based alloy film is prone to peel off from the substrate, and that the property of C-axis orientation has variation due to a subtle difference in the cleanness of the substrate surface. Besides, in a case where the Co-based alloy film is stuck on the metal substrate of Al or the like or on the soft magnetic metal film of permalloy or the like, the crystal orientation of the grains of the underlying metal exerts an adverse effect on the degree of C-axis orientation of the Co-based alloy which is to be adhered to the metal. For example, the materials Al and permalloy have face centered cubic (f.c.c.) crystal structures, and it is known that as the (111) plane of the f.c.c. structure is more parallel to the plane of the substrate, the degree of C-axis orientation of the Co-based alloy to be adhered thereon is better. Since, however, the parallelism of the (111) plane of the actual substrate or soft magnetic metal thin film to the substrate plane is not very good, there is the problem that the degree of C-axis orientation of the Co-based alloy becomes worse than in the case of adhering the alloy film to the plastics or glass substrate. Further, as disclosed in the official gazette of Japanese Patent Application Laying-open No. 56-70618, the degree of C-axis orientation of a Co-Cr alloy film is improved when the Co-Cr alloy film is formed on a substrate which is so prepared that the C axis of a substance having the crystal structure of hexagonal close-packed lattice is oriented perpendicularly to the surface of the substrate. Since, however, the crystal orientation of the underlying substance of the h.c.p. structure is also problematic in this case, the characteristics of the Co-based alloy are governed unfavorably by the property of crystal orientation of the underlying film.
The following references are cited to show the state of the art; (i) Official Gazette of Japanese Patent Application Laying-open No. 56-70618, and (ii) Official Gazette of Japanese Patent Application Publication No. 58-91.